Thermoplastic resins often possess an attractive set of mechanical and physical properties, such as high heat resistance, impact resistance, dimensional stability, high ductility, and optical clarity. Polycarbonates are a good illustration of materials exhibiting many of these attributes. While these properties serve to encourage the use of thermoplastics in many commercial applications, other, less favorable properties often need to be rectified or somehow addressed. As an example, thermoplastics often exhibit low resistance to abrasion and chemical solvents. Moreover, thermoplastic materials are usually susceptible to photodegradation by ultraviolet (UV) light. This type of degradation typically leads to erosion and yellowing of the polymer surface. The discoloration represents a large problem in using products like those based on polycarbonate, which are especially noted for their transparency.
Efforts to alleviate these problems have been undertaken in the past. As an example, coating compositions that include ultraviolet light absorbers have been applied onto thermoplastic substrates and then cured. For the case of polycarbonates, the coating material is often based on a silicone hardcoat matrix, which when cured, provides a good abrasion barrier. The UV absorbing agents incorporated into the hardcoat matrix are often based on benzophenone or benzotriazoles. As described in U.S. Pat. No. 5,391,795 (J. Pickett), various patents disclose this general type of strategy for protecting thermoplastics. For example, U.S. Pat. No. 4,373,061 describes the use of a silicone hardcoat composition comprising benzophenones as the UV absorbing agents, while U.S. Pat. No. 4,278,804 discloses a matrix composition based in part on silanol-reactive alkoxysilyl- or alkanoyloxysilylalkyl ether adducts of aromatic UV absorbing agents. U.S. Pat. No. 5,391,795 itself describes the use of silylated derivatives of 4,6-dibenzoylresorcinols as UV absorbers for hardcoat compositions.
The UV absorbing agents themselves sometimes decompose to some degree, upon exposure to UV light. The decomposition can in turn lead to microcracks in the hardcoat, exposing the underlying polymer surface to the degrading effects of UV light, abrasion, and the like. Thus, the purpose of the hardcoat in protecting the thermoplastic substrate is defeated to some extent.
It's quite apparent that there is a continuing need for new compounds which are effective in providing UV protection for thermoplastics. The compounds should themselves exhibit a high degree of UV stability, i.e., photostability, while still being effective in protecting the substrate. Moreover, the new compounds should be amenable to various forms of use, e.g., they should be capable of being incorporated into a hardcoat composition which is coated onto the surface being protected. The new UV absorbing compounds should also be physically and chemically compatible with the substrate material, and with any type of hardcoat composition being employed. They should not interfere with any of the other properties possessed by the thermoplastic substrate, such as transparency. Furthermore, the new compounds should be relatively easy to manufacture, and their use should not involve an excessive increase in the cost of the thermoplastic product.